The Extreme Sea Surface Temperature and Marine Heatwaves in the Global Oceans in 2023

Globally (90ºS−90ºN) averaged sea surface temperature (SST) was 18.81ºC on March 31, 2023, which broke the historic record (18.78ºC) set on March 6, 2016 after a historically strong 2015−16 El Niño event. The unusualness of the record-breaking SST is that it happened immediately after the triple-dip 2020−23 La Niña event when SSTs were low, while SST record was usually broken after El Niño events. The record-breaking SST raised a lot of medium interests and public concerns on the reasons for the high SSTs, their impacts on climate, extreme weather, and marine environment, as the record SST may increase further as the on-going 2023−24 El Niño event becomes stronger in later 2023 and early 2024. As expected, the new record was broken again and again during July 13−22 (18.82º−18.87ºC), July 27−31 (18.88º−18.94ºC), August 6 (19.95ºC). These record-breaking SSTs are mostly attributed to the record high SSTs in the eastern North Atlantic, northern North Pacific, eastern equatorial Pacific, and the Southern Ocean. The record high SSTs result in active marine heatwaves (MHWs) in those regions during the time when SSTs were in record-high. Our analyses show that two factors are attributed to the series record high SST in 2023: One is the warming SST due to the transition from the triple-dip 2020−23 La Niña event to the 2023−24 El Niño event; the strength and duration of the on-going 2023−24 El Niño event may directly contribute the future record high SST since the globally averaged SSTs are usually high after El Niño events. The other is the overall warming trends of SSTs in the global oceans associated with increasing greenhouse gases.